The epidemic of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) over the past two decades has resulted in the emergence of virulent and multi-drug resistant strains that are causing severe skin infections in healthy people outside of hospitals. These infections are creating a serious public health concern, especially since antibiotic resistance among CA-MRSA clinical isolates continues to rise. Our ultimate goal is to gain insights into the specific human immune responses that could be targeted for more effective vaccination strategies against S. aureus skin infections. This is especially important because all vaccination attempts against S. aureus to date that were previously effective in preclinical mouse models either had no efficacy in humans or actually exacerbated the infection. The recruitment of neutrophils to the site of infection facilitates abscess formation and is required for clearance of S. aureus skin infections. Thus, in this proposal, we will focus on human Th17 cells and responses since these represent antigen-specific adaptive responses that might control PMN recruitment. However, a major impediment to investigating the mechanisms of human neutrophil recruitment is the lack of an in vivo model system with normal human neutrophil numbers. Current humanized mouse models only possess 1-5% of circulating human neutrophils?far below the 45-70% of neutrophils found in human blood. To overcome this we will develop humanized mice with more efficient neutrophil reconstitution to test our overall hypothesis that the human Th17 cell cytokines IL-17A, IL-22 and IL-26 can promote clearance of a S. aureus skin infection by inducing PMN recruitment. We further hypothesize that S. aureus-specific CD4+ T cells, and in particular Th17 cells, can promote neutrophil recruitment and bacterial clearance. In Aim 1, we will determine the mechanisms by which human IL-17A, IL-22 and IL-26 promote neutrophil recruitment and host defense against a S. aureus skin infection in humanized mice. In Aim 2, we will determine the role of human Th17 cells in promoting PMN recruitment and clearing a S. aureus skin infection in humanized mice. Of note, our humanized mouse models will possess both ?matched? human skin and human immune cells, which is highly innovative because this has not been done before to study S. aureus skin infections. Taken together, our proposal will provide new in vivo mechanistic insights by which human Th17 cells and cytokines promote neutrophil recruitment and host defense against S. aureus skin infections. Given that our results will be obtained using optimized humanized in vivo models, our findings are more likely to translate to humans and provide key information about the specific human immune mechanisms to target in the future development of an effective S. aureus vaccine.